1. Field of the Invention
This invention relates to a DC-to-DC converter in which a voltage is derived from the secondary winding of a transformer by appropriately switching a DC voltage applied to the primary winding of the transformer and a DC output voltage proportional to the DC voltage applied to the primary winding can be obtained by rectifying and smoothing the derived voltage.
2. Description of the Prior Art
This type of DC-to-DC converter can enjoy stable operation and also serve as an excellent isolator. In such a converter, however, the current through the primary winding of the transformer is unidirectional so that if the core used in the transformer exhibits a rectangular hysteresis characteristic the core is magnetically saturated in one direction. Consequently, the transformer can no longer perform its proper function and the DC-to-DC conversion operation will be imposssible. Therefore, some means must be provided to reset the core saturated in the one direction. Moreover, if the switching element used in the converter is turned on and off very rapidly, it sometimes happens that when the switching element is turned off, flyback voltages are generated which may damage the element.
Of the conventional reset means the most common is a reset winding provided on the magnetic core of the transformer, through which DC reset current from a DC source is constantly supplied via a resistor. According to this structure, the flux in the core can be automatically cancelled when the current through the primary winding of the transformer vanishes, i.e. when the switching element is turned off, but the constant reset current flowing through the resistor into the reset winding gives rise to heat loss and therefore the problem of heat dissipation. For this reason the power source section cannot be made small in size. This makes unsuitable the provision of the power source in an electronic computer.
The conventional artifice of preventing damage due to the flyback voltage is to cramp the flyback voltage by a circuit consisting of a diode, a resistor and a capacitor so as to make the amplitude of the flyback voltage equal to that of the voltage of the power source so that the switching element may be prevented from being damaged. In this artifice, too, current flows through the resistor of the cramping circuit to cause heat loss.